Internal combustion engines are ideally operated in a way that the combustion mixture contains air and fuel in the exact relative proportions required for a stoichiometric combustion reaction (i.e., where the fuel is burned completely.) A rich-burn engine may operate with a stoichiometric amount of fuel or a slight excess of fuel, while a lean-burn engine operates with an excess of oxygen (O2) compared to the amount required for stoichiometric combustion. The operation of an internal combustion engine in lean mode may reduce throttling losses and may take advantage of higher compression ratios thereby providing improvements in performance and efficiency. Rich burn engines have the benefits of being relatively simple, reliable, stable, and adapt well to changing loads. Rich burn engines may also have lower nitrogen oxide emissions, but at the expense of increased emissions of other compounds.
In order to comply with emissions standards, many rich burn internal combustion engines utilize catalysts, such as non-selective catalytic reduction (NSCR) subsystems (known as 3-way catalysts). Catalysts may reduce emissions of nitrogen oxides such as nitric oxide (NO) and nitrogen dioxide (NO2) (collectively NOx), carbon monoxide (CO), ammonia (NH3), methane (CH4), other volatile organic compounds (VOC), and other compounds and emissions components by converting such emissions components to less toxic substances. This conversion is performed in a catalyst component using catalyzed chemical reactions. Catalysts can have high reduction efficiencies and can provide an economical means of meeting emissions standards (often expressed in terms of grams of emissions per brake horsepower hour (g/bhp-hr)).
In order to achieve low CO and NOx emissions levels, a catalyst must be operated within a relatively narrow operating window that corresponds to a range of air/fuel mixtures. However, the operating window for optimal CO and NOx emissions levels varies in size and location over time as operating conditions at the engine vary. For example, as the environment in which the engine is operated changes (e.g., temperature of area surrounding the engine rises or falls, moisture in the air surrounding the engine increases or decreases, etc.), the operating window may become more narrow or broad and/or drift such that the air/fuel ratios that allow the engine to maintain low CO and NOx emissions levels (e.g., levels below Environmental Protection Agency (EPA) limits) may change. Similarly, as the engine operating conditions change (e.g., temperature of engine rises or falls, quality of fuel changes, etc.), the operating window may become more narrow or broad and/or drift such that the air/fuel ratios that allow the engine to maintain low CO and NOx emissions levels may change. In the current state of the art, regular manual adjustment of the air/fuel ratio for an engine is required in order to ensure that the engine is maintaining low CO and NOx emissions levels.